Must admit, I hate angle and T connectors. From what I understand, RF energy transfers as a energy field along coax. When it meets a 90 degree turn, even if the center conductor is perfectly aligned in the center, the fields do not behave nicely. Inside the angle, is a shorter path than center, and outside the angle, is a longer path. Net result is the input signal is significantly disrupted. Just my experience. Waveguides have similar problems and even coax cables suffer when tight bends are used. Must admit, I love the xx400 cables, large, stiff, minimal bends make them much better carrying UHF signals like our 1090 MHz.
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Your description is spot on and provides additional technical detail as to why such āconveniencesā should be avoided. I failed to mention in my first caution that signal levels where also showing losses as compared to the straight coaxial connections. Thanks for your follow up.
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Ok, I found out why my heatmap plots look so bad. When you run rtl_power the bias-t gets turned off. That turns off the LNA and then youāre just looking at noise from the rtl-sdr dongle front end. As far as I can tell, you canāt turn on the internal bias-t in the rtl-sdr dongle and run rtl_power. You would have to supply an external bias-t voltage to the LNA to turn it on.
Here is a heatmap png where I took out the LNA and just increased the gain in the rtl-sdr dongle to 50 dB. Looks more normal:
You can also tell if the heatmap is going to be reasonable by looking at the lines that heatmap.py puts out when it runs. For the above I got:
loading
x: 4576, y: 60, z: (-34.560000, 4.330000)
drawing
labeling
saving
Here there is a z min to max of about 39 dB. On the previous (noise) plots, z had a min to max of about 3 dB.
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Yes, if you have a bias-t-enabled LNA, the cause is most likely that bias-t gets switched off.
Please use rtl_power -T in gainmap.sh to switch on bias-t, see rtl-sdr-blog/src/rtl_power.c at master Ā· rtlsdrblog/rtl-sdr-blog Ā· GitHub
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So is -T an option in rtl_power to turn on the bias-t ?
The rtl_power --help says:
[-T enable bias-T on GPIO PIN 0 (works for rtl-sdr.com v3 dongles)]
But how does that turn on the bias-t in the rtl-sdr dongle?
I guess I would have to wire up GPIO PIN 0 to the LNA to provide power.
To me that looks more like either a mistake or a leftover from very old times; looking at the code, it just sets the right variable for the call itself. ā Besides, Iām using it over here in my (USB-)setup and it works as I expect.
P.S.: Please remember that GPIO for the RTL-SDR /i.e., ,appendix rtl sdr gpio Ā· rfrht/FT991A-PAT Wiki Ā· GitHub) is not to be confused with e.g., a Raspberry Piās GPIO.
Ok, I was assuming that was the Raspberry Piās GPIO.
So I should be able to just use the -T option in rtl_power to turn on the rtl-sdr bias-t?
Yes, correct, and thatās how I use it.
Youāre right the -T option in rtl_power does turn on the bias-t.
Thanks.
By the way it looks like this same problem with the bias-t came up in July 2019 on this thread (Message 353):
Same setup that I have for 1090. His heatmap plot looked identical to what I had when the bias-t was off.